In the Prototype Fast Breeder Reactor, a core catcher is provided as an in-vessel core debris retention device to collect, support, and maintain in subcritical configuration the relocated core debris generated from fuel melting as a consequence of a severe accident scenario. It acts as a barrier to prevent settling of debris onto the main vessel and helps to maintain the main vessel temperature within acceptable creep range. In the Safety Engineering Division of the Indira Gandhi Center for Atomic Research, model experiments are carried out in water using a geometrically similar model to understand natural convective heat transfer and fluid flow in and around the core catcher below the grid plate. Influences of cylindrical and annular central openings (chimney) through the core catcher assembly are investigated to assess their relative thermal performances. Resistive heating elements are used as heat source to simulate debris decay heat on the core catcher. Series of experiments were carried out with both configurations. Temperatures were monitored at critical positions and compared with numerical evaluation. Flow fields and isotherms are analyzed with a computational model to understand the fluid flow and heat transfer characteristics inside the cavity along with experimental data for specified steady-state temperatures on the heat source plate. Numerical results are found to be in good agreement with those obtained from the experiments. The combined efforts of numerical and experimental work conclude that core catcher assembly with annular chimney is better in terms of natural convection heat removal capability.